A ceramic substrate for mounting a semiconductor device has a metallized pattern to be connected to an electrode of the semiconductor device, formed on a face(s) of the ceramic substrate. Moreover, if the ceramic substrate is used, for example, as a multilayer substrate or a submount, an electroconductive via for conduction between upper and lower portions of the substrate is formed on the ceramic substrate (hereinafter, the ceramic substrate having the electroconductive via and the metallized pattern is referred to as a “metallized via-holed ceramic substrate” in some cases).
As a method for manufacturing the metallized via-holed ceramic substrate, a co-firing method (simultaneous firing method) and a post-firing method (sequential firing method) are known. In the co-firing method, a metal paste layer is formed on an unfired ceramic substrate precursor referred to as a green sheet, or a metal paste is filled into a through-hole formed in the green sheet, to prepare and fire a metallized via-holed ceramic substrate precursor. In this method, the green sheet and the metal paste are simultaneously fired.
In the post-firing method, a metal paste layer is formed on a sintered substrate obtained by firing a green sheet, or a metal paste is filled into a through-hole formed in the sintered substrate, to prepare and fire a metallized via-holed ceramic substrate precursor. In this method, the green sheet and the metal paste are sequentially fired.
Either method allows the metallized pattern to be formed on the ceramic substrate, and the substrate obtained in the method is used as a substrate for mainly mounting electronic components. In the co-firing method, however, the green sheet tends to unevenly contract upon firing, and if, for example, a square green sheet is sintered, a central portion on each side thereof contracts to warp inward slightly, and the substrate deforms into a star shape. Thus, if many metallized patterns and electroconductive vias having the same shape are formed on one green sheet, a slight change in the shape of the patterns and in the position of the vias depending on the location of the patterns is unavoidable. Moreover, in the co-firing method, the green sheet and the metal paste are simultaneously fired at high temperature, and thus, the co-firing method has such a disadvantage that it is necessary to use a high-melting-point metal paste of molybdenum, tungsten etc., as the metal paste, and that other metals excellent in conductivity cannot be used.
On the other hand, in the post-firing method, the metal paste layer is formed on the sintered ceramic substrate, or the metal paste is filled into the through-hole formed in the sintered ceramic substrate, to fire the metal paste, by which the metallized pattern and the electroconductive via are formed. Upon firing of the metal paste layer, the metal paste layer contracts in its thickness direction, but hardly contracts in its plane direction. Thus, there is not any problem that the pattern shape varies depending on the location, as is seen in the co-firing method.
However, since the metal paste itself does contract, contraction of the metal paste in the through-hole occurs upon sintering, thus causing voids in the formed electroconductive via, and making it difficult to form a dense via.
Patent document 1 discloses that a titanium layer and a copper layer are formed, by sputtering, on a ceramic substrate having a through-hole and are then plated with copper to form a wiring pattern and an electroconductive via. Since the method requires the step of sputtering, a manufacturing facility for the sputtering is required, and the method does not allow simple manufacturing of the metallized via-hold substrate. Moreover, in filling copper into the through-hole by plating in this method, if the diameter of the through-hole or the thickness of the substrate increases, more time will be required for the filling, causing degradation in the productivity. In this point, the method still needs improvement.